1. Field of the Invention
This invention relates to an improved vent valve for a gas strut, and to a gas strut incorporating such a vent valve.
2. The Prior Art
Gas struts are extensively used for the purpose of applying a force between mutually moveable components of a structure. A typical use of gas struts is to balance, at least partially, the weight of a lid, cover or door to assist manual raising of a lid, cover or door.
In many applications, particularly where the struts are to be used in mass produced products such as automobiles, the force which a particular strut is required to produce will be known at the time when the strut is manufactured, and the strut can accordingly be charged, at the time of manufacture, with the correct gas pressure to produce the required force. With such struts there is no requirement for subsequent adjustment of the force exerted by the strut.
In some applications, however, it is desirable to be able to adjust the force exerted by the strut after manufacture. In order to achieve this, it is known to charge struts, during manufacture, with a higher gas pressure than will ultimately be required when the strut is in use, and to provide the strut with a vent valve which enables some of the gas charge in the strut to be vented subsequent to manufacture in order to tune the force which the strut produces to the particular application for which it is being used. A gas strut having such a vent valve is described in GB-A-2168453.
Whilst the gas strut of GB-A-2168453 discloses the principle of a vent to enable the gas pressure within the strut to be selectively decreased subsequent to manufacture, the particular arrangement proposed in GB-A-2168453 suffers from two significant disadvantages.
Firstly, it will be appreciated that after the adjustment of the gas pressure within the strut has been effected it is imperative that the vent valve seals absolutely the vent passage to prevent further escape of gas. The total charge of gas within the gas strut is quite small, and the gas is confined at a high pressure. Any leakage of gas will result in a decrease in the force which the strut is capable of applying, and this is highly undesirable. In this context, it must be remembered that a strut may be required to have a service life of ten years or more. With the vent valve arrangement disclosed in GB-A-2168453 the use of a screw having a conical point which seals, in use, with a conical seat makes achieving the necessary seal difficult. Considerable care must be exercised in the manufacture of the seat and the conical end of the screw to ensure an absolutely gas tight seal.
The second problem associated with GB-A-2168453 is that the venting of excess pressure which occurs when the vent valve is open is difficult to control. As soon as the conical face of the control screw moves out of engagement with its corresponding conical seat a relatively large area is available for gas flow, and this may produce an excessively fast release of gas pressure. Further, since gas pressure in the case of GB-A-2168453 is vented via a drilled passage in the vent valve body the release of gas pressure may produce a blast of gas which may impinge on the hands or face of the worker adjusting the gas pressure. Since the venting gas may be very cold as a result of the adiabatic expansion which occurs when it leaves the gas strut, it is highly undesirable that the venting gas should be directed, as a blast, at any part of the body of the person adjusting the gas pressure.
We have now devised an improved vent valve which substantially eliminates the problem outlined above.